Data transfer

ABSTRACT

A sub-sea oil pipeline installation comprising a production platform (10) and sub-sea facilities (15) at a plurality of well-heads (16), has a data transmission system by which data is transmitted in either direction between the master platform (10) and the sub-sea facilities (15) via communication channels formed by the electrically conducting material of the pipelines (12, 14). The data is transmitted in the form of a VLF or ELF electro-magnetic signal which comprises changes of voltage level oscillating about the DC voltage level of the pipe (12, 14) so that the mean level of the signal is the DC voltage level of the pipe (12, 14).

DESCRIPTION

This invention relates to a pipeline system comprising a metal pipelinehaving a coating of insulating material which, in conjuction withsacrificial anodes electrically connected to the pipe, provides cathodicprotection, there being signal generating means adapted to he coupled tothe pipeline and operable when so coupled to apply data to betransmitted in signal from to the pipeline for conveying along anelectrically conducting channel formed by the metal pipeline.

U.S. Pat. No. 3,129,394 discloses the use of a pipeline to transmitalternating current signals. The pipeline is not used as a wire orconductor as such. It is used as the inner conductor of a coaxial cable.The insulating wrapping (provided for corrosion protection) provides thedielectric and the ground surrounding the pipe acts as the otherconductor. By employing the pipe and ground as a co-axial cable,attenuation is substantially reduced and useful signals can betransmitted over substantial distances by amplification of the signal atone or more intermediate stations. Due to dissipation by leaks in thepipe wrapping, and other reasons, for most of the frequency rangestaught it was considered necessary to use higher output power than mostcommunication systems. There is a fear that that will be undesirablebecause it will interfere with the normal direct current cathodicprotection of the pipeline. However the possibility of using signals inthe audio range is hinted at. It is said that the use of the pipe andsurrounding earth as an alternating current transmission means will notinterfere with the normal direct current cathodic protection of thepipeline but there is no disclosure of how this was achieved.

U.S. Pat. No. 3,551,890 discloses a system wherein a metal pipelineforms an electrically conducting channel for transmission of data, therebeing signal generating means adapted to be coupled to the pipeline andoperable when so coupled to apply data to be transmitted in a signalform to the pipeline for conveying along the channel. In carrying outthe teachings of that reference data is transmitted by modulating eitherthe amplitude or the frequency of an AC carrier signal. We have foundthat no useful signal could be detected at the end of a pipeline whichwas 4 km long using such a technique. In practice it is desirable totransmit data over considerably greater distances than that in suchpipeline installations.

An object of this invention is to provide a pipeline system with meansfor transmitting data over considerably greater distances withoutinterference with the cathodic protection of the pipeline.

According to this invention there is provided a pipeline systemcomprising a metal pipeline having a coating of insulating materialwhich, in conjunction with sacrificial anodes electrically connected tothe pipe, provides cathodic protection, there being signal generatingmeans adapted to be coupled to the pipeline and operable when so coupledto apply data to be transmitted in signal form to the pipeline forconveying along an electrically conducting channel, formed by the metalpipeline, wherein the signal generating means are operable to generateas the signal a pulsed electro-magnetic signal which comprises changesof voltage level in a very low frequency range (VLF and below), andwherein means are provided which are operable to use data transmittedvia the channel provided by the pipeline to monitor the condition of theinsulation of the pipeline. Preferably the signal generating means areoperable to generate the electro-magnetic signal in the very lowfrequency range (VLF and below) so that it comprises changes of voltagelevel oscillating about the DC voltage level of the pipeline so that themean level of the signal is substantially the DC voltage level of thepipeline. The longer the pipeline that forms the channel the lower thefrequency that would be used. For the longer pipelines frequencies inthe extremely low frequency (ELF) range (3 to 300 HZ) would be usedwhereas for short pipelines frequencies in the VLF range (3 to 30 kHZ)could be suitable. The available data bandwidth depends on constraintsimposed by the physical situation for any particular application. Forthis reason, provided adequate bandwidth is available, any form serialdata signal can be transmitted including analogue (e.g. video) and timeencoded digital signals.

In an endeavour to avoid the use of the traditional umbilical cablewhich has been used for many years in the sub-sea oil pipeline industryfor two-way transmission of telecommunications between a master stationon a production platform and each of a number of sub sea facilities atwellheads, amongst other services such as the supply of hydraulic powerto the sub-sea facilities and the supply of chemicals for injection intothe fluid to be conveyed from the well-head through the pipeline, apartfrom the proposals of Long and Silverman in U.S. Pat. No. 3,129,394 andU.S. Pat. No. 3,551,890 referred to above, it has been proposed thatunderwater acoustics be used to transmit data between the master stationand the sub-sea facilities through the sea. Such a proposal has a numberof drawbacks. Thermal barriers at varying depths in the sea would causerefraction of the signal resulting in multi path fading and completeloss of signal. To achieve the range requirements, transmitter powers inexcess of 1 KW would be required. Such a system would be expensive andcould only support low data rates.

A preferred application of this invention is to sub-sea oil pipelineinstallations for the transfer of data between sub sea facilities andtheir production platform, the data transfer comprising instrumentationor status information from any point on the pipeline installation, usebeing made of an actual pipe carrying the fuel/water between a well-headand the production platform as the pipeline which forms the electricallyconducting channel.

The application of this invention to sub sea oil pipeline installationswhich do not require a supply of chemicals for injection into the fluidto be conveyed from the wellhead through the pipeline enableselimination of the traditional umbilical cable which has long beendesired. Such an application of this invention is more satisfactory thanearlier proposals to achieve that end. Other arrangements can readily bemade to meet any requirement for power at the sub-sea facility so thatthe need for a hydraulic supply can be avoided.

The levels of safety and reliability of the preferred embodiment of thisinvention are high because buried steel pipeline is a robust structuralmedium for signal connection of relatively low impedance. Because ofphysical constraints, over long distances the available data rate islikely to be low. However, in the preferred application of thisinvention, timeliness of data is not critical and would allow the use offorward error detection and correction signal encoding to provide highdata integrity without noticeable time delay to the user.

The invention takes advantage of the current techniques used forinsulation of the metal pipelines by coating them with an insulatingmaterial to prevent corrosion of the pipe. Application of the inventionmust take account of and therefore co-exist with active or sacrificialcathodic protection. These measures enable a prediction of the energyloss through regions of the pipeline where the insulation is absent ordegraded such as weld joints. It is in that way that the invention isadapted as a means for monitoring the condition of the pipeline.

The invention is also applicable to the so-called `flexible` pipelineconstruction.

No direct current leak paths are introduced by the application of theinvention to a pipeline.

The range of effective operation of the invention is thought to belimited to a distance (say 35 KM) within which the VLF or ELF signal andthe cathodic protection of the pipeline may coexist on the pipelinewithout the level of such protection being reduced. VLF or ELF signalsare used because the characteristic of the pipeline acts as a low passfilter with acceptable attenuation for the range of effective operation.

In order to minimise the impact of signal modulation on the cathodicprotection voltage, it is desirable to arrange for the signal potentialto sum to zero and for transmission to take place only in short burstssuch that only small variations in potential are seen on the pipeline.

In the preferred embodiment, signalling is managed by a master stationlocated on the platform, and takes place in one of three modes, viz.normal data transfer, special data transfer and alarm interrupt. Normaland special data transfers are initiated by the master station. Alarminterrupts are initiated by any sub-sea unit which detects an alarmcondition. All signal sequences conveniently incorporate an interrupttime domain to facilitate near real time transfers of the alarminformation.

Data flow is half-duplex in operation of this preferred embodiment ofthe invention. The master station manages communication between theplatform and the sub-sea data transfer units (DTUs). In so doing ittransmits synchronisation, communication management and configurationinstructions to the sub-sea DTUs. The sub-sea DTUs transmit time-taggedtelemetry data, health monitoring data and alarm data items to themaster station.

In the preferred embodiment, a transmitting DTU, either at the platformor at a sub-sea facility is operable to launch a signal onto thepipeline, the frequency band of which is selected to avoid known EMnoise and the amplitude of which is sufficient to overcome ambient noiseto allow detection at the receiving DTU. The signal is received by allthe DTUs on the pipeline. Onto this signal, the transmitting DTU imposesa digital data stream.

Conveniently a DTU will only respond to two signal types, viz. thoseuniquely addressed to it and those globally addressed to all DTUs on thenetwork. The signal content is extracted from the pipeline EM noise anddecoded to provide the data items. The transmitting DTU then fallssilent in readiness to receive requested data.

Normal data transfer is an automatic procedure which interrogates eachDTU on the network and logs their telemetry data for use on theplatform. It is proposed that a complete DTU data log will betransferred to the platform in 60 seconds, with the objective ofachieving an undetected bit error rate of 10⁻⁹.

Special data transfer is a manual intervention with the system, andenables direct operator communication with its selected DTU whereby anoperator can configure and control any sub-sea DTU.

A sub-sea DTU which detects an alarm or a trend which will lead to analarm condition in its transducer array, will transmit an alarm identityduring the interrupt time domain of any ongoing transmission.

Preferably all transmissions are curtailed by the alarm interrupt code.

In the event that more than one DTU senses an alarm condition, themaster station lists the DTUs which have raised an alarm and schedulesthe transfer of their alarm data for logging and display. An audio tonemay be provided to alert the operator that an alarm condition isdetected.

To expedite the transfer of this alarm data, it is envisaged that thenumber of data items transferred during an alarm condition would beminimised.

Signal extraction from the pipeline EM noise may be achieved by use ofparity bits and proprietary error detection and correction (EDAC)techniques, which enable the receiving DTU to assess the integrity ofthe data.

If integrity is poor, then the receiving DTU may be arranged toautomatically request repeat of the data.

A thermoelectric generator may be provided for sub-sea locations whichdo not have mains power available. Such a thermoelectric generator isparticularly suitable for use at oil production wells. The powerrequirements of the sub-sea elements of a system in which the inventionis embodied are low and that allows construction of a practicalthermoelectric generator with high levels of redundancy. A suitablethermoelectric generator has no moving parts and employs solid statearrays of thermoelectric devices which are sealed using epoxy forsub-sea installation. The external surface of the generator may besealed by an uninterrupted plastic skin to avoid corrosion or leakageproblems. Two configurations of such a power generation unit areproposed, viz. in-line or clamp-on. These are functionally similar.Rather than such a thermoelectric generator, a flow turbine generatormay preferably be provided for use at a water injection well.

Conveniently, a DTU provides the processing capacity to:

1. Manage the system start up after any extended dormancy period;

2. Sample the transducer array and identify alarm conditions;

3. Receive data communications from the master station;

4. Decode telemetry requests from the master station;

5. Transmit encoded telemetry data on request or alarm conditions onoccurrence to the master station; and

6. Provide power conditioning and distribution.

A memory array within the processor may provide a circular buffer inwhich 400 data items from each of 32 transducers may be stored betweendata transfers. The sampling rate and transducer ranges may beconfigurable from the master station.

In the preferred embodiment, a pipeline interface is required at theplatform and at any location to be instrumented. This function requirespre-amplifier, modulator/power amplifier and signal processing to allowfor half duplex communication over the pipeline. Using transformer orother suitable coupling, the interface launches the carrier signal anddata stream onto the pipeline.

To complete the signalling circuit, the DTU may exploit the earthconnections to the well, and a sea return at the production platform toprovide a low impedance return path. Alternatively a dedicated sea waterelectrode may be required to avoid potential corrosion problems at theplatform.

The data could be data concerning conditions at a down hole location ofa sub-sea well and could be transferred from that location alongstructure of the production string of that well.

One embodiment of this invention, and a possible modification of afeature of that embodiment, will be described now by way of example withreference to the accompanying drawings, of which:

FIG. 1 is a block diagram of a sub-sea oil pipeline installationincorporating a data transfer system in which this invention is embodiedfor the transmission of data between a master station on a productionplatform and a sub-sea installation at a wellhead;

FIG. 2 is a block diagram illustrating the system illustrated in FIG. 1applied to a sub-sea oil pipeline installation wherein the masterstation on the production platform is connected to sub-sea installationsat a number of wellheads;

FIG. 3 is an illustration of the system shown in FIG. 2 with currentleakage and return paths depicted thereon;

FIG. 4 is a circuit diagram of a pipeline equivalent circuit between amaster station on a production platform and a sub-sea installation at awell head of the system shown in FIG. 1;

FIG. 5 is a block diagram illustrating the transmit and receive datacoding employed in operation of the system shown in FIG. 1;

FIG. 6 is a block diagram of the arrangement of the master station ofthe system shown in FIG. 1;

FIG. 7 is an operational flow diagram for the master station shown inFIG. 1;

FIG. 8 is a block diagram of a data transfer unit of the system shown inFIG. 1;

FIGS. 9 and 10 together comprise an operational flow diagram of thesystem shown in FIG. 1;

FIG. 11 is a block diagram illustrating the power conditioning and powerdistribution arrangement for a sub-sea data transfer unit located at awellhead of the system shown in FIG. 1;

FIG. 12 is an elevation of a data transfer unit of the system shown inFIG. 1;

FIG. 13 is a section on the line XIII--XIII in FIG. 12;

FIG. 14 is a partly sectioned side elevation of one form of powergeneration unit for use at a sub-sea installation of the system shown inFIG. 1, the part shown above the centre line being in section and thepart shown below the centre line being in elevation but partly cut awayto reveal hidden detail;

FIG. 15 is a view in perspective of another form of power generationunit for use at a sub-sea installation of the system shown in FIG. 1;

FIG. 16 is a diagram illustrating a data transfer system memory of thesystem shown in FIG. 1;

FIG. 17 is a view in elevation of an assembly of a data transfer unitand a power generation unit for installation at a sub-sea facility ofthe system shown in FIG. 1;

FIG. 18 is a block diagram illustrating installation of apparatus inwhich this is embodied on a sub-sea well production string at a downhole location; and

FIG. 19 illustrates electrical current leakage and return paths in theapparatus shown in FIG. 16.

FIGS. 1, 2 and 3 show a production platform 10 which supports a masterstation 11 of the data transfer system, a pipeline 12 connecting theproduction platform 10 to a manifold 13, and five branch pipelines 14A-Ewhich each connect the manifold 13 to a respective tree 15A-E of arespective one of five wellheads 16A-E. An electrically isolating pipecoupling 19 electrically isolates the pipeline 12 from the productionplatform 10. A short length of pipe 18 which leads from the pipecoupling 19 into the production platform 10, is electrically coupled tothe sea at 17. Each branch pipeline 14A-E is electrically isolated fromthe respective tree 15A-E by a respective electrically isolating pipecoupling 21. The short length 22 of pipe leading into the respectivetree 15A-E is grounded to the earth through the well casing of therespective wellhead 16A-E at 23.

The Master Control Station 11 which is shown in block schematic in FIG.6 comprises 3 main parts: viz. a control unit 24, a Power Supply 25 anda DTU 26.

The control unit 24 is based on a high performance IBM PC compatibleindustrial computer. FIG. 6 shows the control unit 24 includes a display27, CPU 28 and keyboard 29. Those elements of the control unit 24together with disc drives and expansion slots are mounted in ananti-vibration rack within an environmental protective enclosure. Thisensures adequate environmental protection during storage, transport anduse.

The operating software is resident on the hard disc with userconfigurable files held on floppy discs.

Each downpipe 12 from the platform 10 will require a separate DTU 26.Communication with these DTUs 26 is by RS 422 link 31 or, if necessary,in harsh electrical noise environments, by optical fibre. Provision ismade for a minimum of 8 serial duplex lines using an interface cardfitted in an expansion slot of the PC 24.

The following system control functions are available using the controlunit 24:

Special data transfer to provide telemetry data immediate request;

Normal data transfer to provide continuous sequential telemetry request;

Sample rate selection;

Time-tagged data logging (onto floppy disc);

Replay of data logged telemetry from floppy disc;

Selection of "System Status" or "Telemetry" data;

Display an alarm message if any parameter is outside limits;

Range limit setting for alarm function;

Modify telemetry sample order and rate; and

Provide additional system outputs.

The control unit display 27 has three selectable screens as follows:

Command menu;

System status;

DTU supply voltage;

Link margins;

TEG differential temperature; and

DTU diagnostic data; and

Telemetered parameters.

The control unit software is written in a high level language such as"C" in an MS DOS operating system environment. The programme willauto-start to prevent tampering or use of the computer for anotherpurpose by personnel. The use of "C" language allows rapid programmedevelopment whilst maintaining excellent peripheral drive capability.

The software is structured using an executive module to schedule othermodules sequentially with interrupt driven modules taking priority whenneeded.

The Master Station 11 is designed to operate from mains supply. Toaccommodate conditions where this supply is intermittent,Uninterruptible Power Supplies (UPS) 32 and 33 are provided for both thecomputer equipment 24 and the DTU 26.

The UPS 32,33 is sized to provide greater than 60 mins operation in theevent of power failure.

A high performance filter is incorporated to protect against mains borneinterface corrupting operation of the computer.

A separate pipeline interface 34 is provided for each pipe network 12,14connected to the platform 10. The pipeline interface 34 incorporates therespective insulation joint 19 and the signal launch and capture takeplace by a coupling 35 at the respective insulation joint 19. Thiscoupling 35 may be achieved by either direct connection to the pipe,capacitive coupling with the pipe or by use of a transformer with thepipe itself acting as one winding thereof.

The pipeline 12,14 is usually provided with sacrificial cathodicprotection anodes at spaced intervals and the remainder, apart from thearea of weld joints, is coated with an insulating material to preventcorrosion of the pipe. The system will be arranged to overcome signallosses caused by up to 5% of the pipe external area being exposedelectrically to the sea water. The anodes bias the pipeline 12,14 at alower potential than the sea. The pipeline 12,14 can be regarded as asequence of discrete stages, each containing a single anode and lengthof pipeline as is illustrated in FIG. 3. Each stage has an upstream anda downstream boundary. The initial boundary is at the platform end DTU26 and the final boundary at a sub-sea DTU 36. Each stage contains a runof pipeline with self inductance and resistance elements distributedabout a sacrificial anode.

The anode and the area of pipeline material exposed to the sea arerepresented by capacitive and resistance elements between the pipelineand the sea or earth paths. These distributed earths represent aconstant potential above that of the pipeline itself, due to thecathodic protection. Within a single stage small circuits exist betweenthe anode and the exposed material. The area of influence of the anodeis substantially wholly within the boundary of the respective stage, sothat the rate of erosion of the anode is effectively dependent only onthe area of the exposed pipeline within the stage, a steady statepotential difference with high current density being provided.

The purpose of the DTU 26 is to provide an intelligent interface betweenthe Master Station 11 and the pipeline 12. Serial data is transmittedfrom the Master Station 11 to the DTU 26 over the RS 422 hard wire link31. This is then formatted for transmission of telemetry request to aparticular sub-sea DTU 36. This request is transmitted via the pipeline12,14. The DTU 26 then waits for a reply which, when received, isre-formatted and transmitted back to the Master Station 11 over the RS422 serial link 31.

It will be understood that, in order to deal with signal leakage lossesand induced electrical noise, signal formatting and encoding elementsare provided at the transmission end and signal recovery and decodingelements are provided at the receiving end of the pipeline 12,14.

Functionally, the Interface 34 is identical to a sub-sea version 37described below. The DTU 26 is similar (if not identical) hardware tothe sub-sea version 36. However, the software is substantiallydifferent.

If required, the control unit 24 can be fitted with interfaceelectronics 38 to provide serial, digital or network communications tosuit individual user needs.

The purpose of such connection is to provide dissemination of data. Theinterface circuitry to achieve this function is of proprietary type andis installed in the expansion slots provided in the control unitcomputer. The software driver is custom written to suit.

The sub-sea installation comprises the following equipments--the sub-seaDTU 36 mentioned above, a power generation unit (PGU) 39 and atransducer cable assembly 41.

The sub-sea DTU 36 provides the following functions:

Reception and decode of telemetry request signals;

Transmission of encoded telemetry;

Identification and transmission of alarm data;

Continuous sampling of transducer parameters;

System health monitoring; and

Maintenance of comprehensive data log.

A block diagram of the DTU 36 is presented in FIG. 8 and individualelements are described below.

The DTU 36 consists of a heavy walled steel main-tube 42 having aclosing plate welded to its upper rim as shown in FIGS. 12 and 13. Aside mounting plate structure 43 is welded to the tube 42 approximatelyhalfway along its length.

A large, multi-pin, sub-sea mateable connector 44 is located near thelower end of the main tube 42. When installed, a special site dedicated,moulded cable assembly 41 (including branches to the power generation,flow line tap-in studs and transducer connectors) is mated with thisconnector 44.

The lower end of the main tube 42 is closed by a removeable endplate 46which is bolted to the rim of the tube 42. Four lugs 47 at the peripheryof the endplate 46 satisfy the end mounting requirement. The holecentres are identical to those of the side mounting plate 43, thusaffording flexibility in mounting.

The end plate 46 is sealed to the internal surface of the tube 42 by an"O" ring 49 and includes four anti-vibration shockmounts 51 on itsinternal face. These provide mounting points for the electronics chassis52 and electrically insulate the latter from the main tube structure 42.The chassis 52 includes mountings for a pre-amp 53, modulator/amplifier54, a signal processor 55 and an electronics unit 56. The latter unit 56consists of a rack containing half-size "Euro-cards" having thefollowing functions:

1. Power conditioning (see FIG. 11) (3 cards);

2. CPU and battery powered clock (1 card);

3. A/D and OPTO insulation (1 card); and

4. Digital I/O with OPTO insulation (1 card).

The electronic chassis 52 is stabilised and supported at its upper endby means of a moulded polyethylene foam packing 57 fitted between thechassis 52 and the inside of the main tube 42.

Four sealed gas-recombination batteries 58 are mounted in a tray 59 andform the main battery pack 61. The pack 61 occupies the upper portion ofthe main tube structure 42 and is separated from the lower electronicsbay by a "pressure" bulkhead 62.

The pack 61 is mounted in, and protected by, polyethylene foam packers63.

The pressure bulkhead 62 seals off the battery bay so that theelectronic components 53-56 of the DTU 36 are protected. Any pressurerise may be detected by a pressure transducer mounted on the upper faceof the bulkhead 62. A pressure rise would be caused by the batteries 58being charged at a rate high enough to cause "gassing", which is a mostunlikely occurrence. The rate of charge will always be regulated belowthe gas step. The bulkhead 62 is sealed by an "O" ring and is retainedby a split ring which locates in a groove 64 in the main tube wall. Thesplit ring is locked in its groove 64 by means of a bolted block.

The sub-sea DTU software is written in modular form to enable thoroughtest prior to integration. This modular approach allows access to areasof code to provide modifications to suit particular requirements (userconfigurable).

The software uses an executive module to schedule other modules, exceptany interrupt driven modules. A main loop 65 is scheduled to run every50 ms. The flow chart in FIGS. 9 and 10 illustrates logical operation ofthe DTU 36.

The executive module schedules the activities of all synchronousfunctions in the programme.

An input module is scheduled by the executive and acquires data from thedigital I/O interface.

An analogue input module acquires a maskable number of analogue inputs,scales and offsets them, then stores into internal and external RandomAccess Memory (RAM) for use by other modules. The mask, scale and offsetvalues are user configurable.

A real time de-bug monitor module is provided for use duringdevelopment, test and evaluation. This monitor is standard software andallows real time monitor and modification of memory locations.

Depending on circumstances, the DTU 36 will operate in different modes,e.g. data link valid, link lost, sync search, data request etc. A logicmodule defines user configurable, logical operation of the DTU 36 foreach of these modes.

The logic module is activated by a "signal detected flag" from thesignal processor 55. Control of the signal processor 55 is theninitiated to synchronise with the incoming data.

A Universal Serial Asynchronous Receiver Transmitter (USART) isinitialised by an initialisation module to look for a pre-determined twobyte header of a telemetry request frame. Once recognised, the USARTinterrupts the processor 55 and initiates the data validation procedure.

If the telemetry request is valid then the appropriate data istransmitted back to the master controller 24 under control of atelemetry send module. This process is illustrated by the flowchart inFIGS. 9 and 10.

A timer module 48 interrupts the processor 55 every 50 ms to set flagsetc and update timer counters for use by other modules.

Following a telemetry request, a telemetry send module loads the USARTwith data, which is then sent via the pipeline interface 37 to themaster controller 24. The transmitted data uses a clock derived from thetelemetry request signal.

If the Master Controller 24 does not receive data following a request itwill try again automatically. It will continue this process for alimited number of tries before indicating communication link failure.

A diagnostics module checks the various areas of memory, and returns anerror code for display, if a device is not working. In that event, therest of the programme, except the monitor, does not run.

All RAM, and ROM, are tested with a "walking" bit pattern to test eachbit of system memory.

A telemetry assembler module assembles data in the correct format inaccordance with user defined tables. A buffer is used to store the datafor use by the telemetry send module.

All peripherals and some RAM are initialised into a defined state beforethe DTU 36 is run. This task is performed by an initialisation module.

All global variables and constants are defined in the MAP file. Look-uptables and reserved memory areas are defined in the MAP file.

Current transducer values are monitored against user defined alarmranges and transducer output trends to establish possible future alarmconditions. If an alarm condition is identified an alarm detectionmodule interrupts the processor 55 and enables the telemetry/send moduleto transmit the alarm interrupt coding.

Timing of the alarm telemetry is scheduled for the interrupt timedomain.

The Data Transfer system requires a source of power at the well 16 tooperate its electronics. In the absence of an umbilical, the thermalelectric generator 39 (TEG) is fitted directly to the pipeline (14) as ameans of power generation local to the well 16.

The electrical power from the thermal electric generator 39 is providedby conversion of heat to electrical energy. Heat is provided by placingthe generator 39 between a heat source and a heat sink. For thisapplication the heat source is a flowing oil in the pipeline 14 and thesink, sea water.

Output from the PGU 39 is conditioned for use in the DTU 36 as shown inFIG. 11.

FIG. 14 shows one form of thermal electric generator 39 which isconstructed as a pipeline stage. It has a bolted flange interface beingprovided with standard flanges 66. The PGU 39 is designed to generateapproximately 250 watts across an oil/ambient temperature difference of70° C.

Apart from the guard assembly, the PGU 39 is a one-piece steel unit.

Electrically, the unit 39 consists of four groups of five banks ofThermo-Electric Devices (TED) 67 with each bank having twelve devices ina longitudinal row.

Mechanically, this results in the requirement for twenty longitudinalrows of twelve devices 67 mounted on the surface of the unit 39.Therefore, twenty longitudinal facets with inter-facet wiring groovesare provided to mount the devices 67. The grooves are blind at one endand run into an annular wiring gallery at the other. The gallery alsoprovides wiring access to the sub-sea mateable connector 68 which ismounted within a local flat area on a 45° conical face adjacent to thegallery.

Each TED 67 is bonded to its respective facet using a heat transmitting,strain tolerant adhesive. The two electrical leads (per TED) arearranged to lie in the wiring grooves such that when connected together,the wiring is submerged in the groove. The TEDs 67 are also arrangedwith a small longitudinal gap between each unit.

With the connector 68 installed and wired into the gallery, all grooves,gaps and galleries are "potted" using an epoxy resin up to the surfaceof the TEDs 67. A continuous plastic film 69 is applied all over theactive area and is sealed to annular "lands" provided at each end.

Finally, the unit includes a protective guard which is formed by tworobust annular discs 71 and 72, one at either end of the active area.The discs 71 and 72 are connected and retained by steel rods 73 whichare held in place with stiffnuts 64, thus forming a cage 75. The cage 75provides a guard for the active area.

FIG. 15 shows an alternative to the in-line PGU 39 described above withreference to FIG. 14. It is a segmented "bracelet" configuration powergeneration unit 76. This unit 76 will enable electrical power generationto be achieved without breaking into the flowline 14 as the unit 76 maybe "wrapped" on to the flowline 14 retrospectively. The unit 76 is aclamp-on arrangement for installation between the tree 15A-E and thepipeline 14.

Two of these units would be required to produce 250 W although theircombined length would be similar to a single "in-line" unit 39 asdescribed above with reference to FIG. 14.

The basic principles of construction follow those used in the in-linePGU 39 except that they are applied to two separate clamp-on halves 77and 78. The halves 77 and 78 include endplates 79 which include featureswhich permit hingeing 81, clamping 82 and inter-half, flexibleelectrical connections.

The most significant detail design change as compared to the PGU 39described above with reference to FIG. 14 applies to the "bore" of theunit which is clamped to the flowline 14 Each semi-bore 83,84 is groovedlongitudinally and circumferentially so that approximately 21/2 cm.square islands are produced. A flexible seal is attached to theperimeter of each semi-bore 83,84. A gallery connects the grooves to aconnector mounted in one endplate.

The purpose of the above features is to enable the implementation of anassembly technique designed to eliminate the potential crevice corrosionbetween flowline 14 and the PGU bore. This is best illustrated bydescribing a typical installation sequence.

The unit 76 is initially loosely clamped to the flowline 14 such thatthe semi-bore seals are in light contact. At this point, a corrosioninhibitor cartridge is fitted to the connector and a quantity of theinhibiting liquid is injected into the space between the flowline 14 andthe PGU 37, mixing with and displacing the sea water past the seals.

When a minimum level of inhibitor mixing has been exceeded in thecaptive chamber, the cartridge is removed and the clamps 82 aretightened until the PGU semi-bores 83 and 84 are in hard contact (inplaces depending on local tolerances) with the flowline 14. Theinhibiting mixture is expelled past the seals (which are designed topermit outward flow with sufficient pressure differential) so that, oncompletion of clamp-up, non-contacting areas will be filled with thecorrosion inhibiting mixture.

Finally, an inhibitor top-up and expansion compensation cartridge isfitted in place of the initial priming cartridge. This cartridge allowssmall flows of inhibiting fluid in and out of the clamped region tocompensate for temperature change and also provides a slightoverpressure so that in the event of seal leakage a measure of "top-up"would be afforded. However, even if the cartridge 85 were exhausted dueto a small seal defect, allowing leakage, the rate of dilution of theinhibited mixture within the clamped region would be very slow due to anominally zero pressure differential and a small connecting passage inthe seal. It should also be noted that the material chosen for the PGU39 would be the same as the flowline 14 to which it would be clamped,thus eliminating galvanic corrosion.

In operation of the preferred embodiment of the invention, a VLF signal(3-30 kHZ) or more especially an ELF (3-300 HZ) is induced on to anelectrically isolated section of the pipe 12,14 (see FIGS. 3 and 4). TheVLF or ELF signal applied to the pipe 12,14 is a pulsed electromagneticsignal which comprises changes of voltage level oscillating about the DCvoltage level of the pipe 12,14 so that the mean level of the signal isequal to the DC voltage level of the pipe 12,14.

A loop then exists with current flowing through the pipeline structureand returning through the earth path/sea path returns. The loop isimperfect and current leak paths exist between the pipeline structureand the sea/earth returns in regions where metal surfaces are exposed,such as cathodic protection bracelets. The current flow is dividedbetween the branch pipeline 14A-E at the manifold 13 and the signal isattenuated accordingly. Between each boundary between juxtaposedpipeline stages, as described above with reference to FIG. 3, theleakage and loss mechanisms modify the signal and establish the entryboundary conditions for the subsequent stage. The VLF or ELF signal isrepresented by a small potential imposed on one boundary at a very muchlower current density than is typically provided by the influence of therespective anode.

Transmission is half-duplex between stations at the wellheads 16A-E andat the platform 10.

Input power limits and theoretical data rates depend on factors such asline inductance, line capacitance, line leakage (insulation integrity)and induced electro-magnetic noise values.

For low information rates of 50 bits per second or less, high integritycommunication can be achieved over extended ranges without degradationof the cathodic protection.

FIGS. 1 to 3 show a pipe transmission line with earth and sea pathreturns plus signal input and signal capture systems at either end.

The DC resistance of the pipeline 12,14, even for 35 km, is very low(typically <1 ohm) given good electrical conductivity through pipejoints which can be reasonably expected if most joints are welded.

Many alternative signalling techniques could be applied since the choiceto be made depends heavily on the conditions found in a practicalsituation.

Coding is applied to a carrier signal or basic link in order to minimiseundetected bit error rate, and to facilitate extraction of a low energysignal from uncertain (and possibly variable) electro-magnetic noise.

A block diagram showing the proposed coding/decoding strategy is givenin FIG. 5.

It is envisaged that the system hardware will be largely common to allinstallations. However, specific parameters in the operating system willbe configured for the particular installation. Two analyses areenvisaged:

Pipeline electromagnetic noise spectral analysis; and

Pipeline configuration, length and interfaces.

The spectral analysis may be empirical for retro-fitment of the systemon existing installations and would be theoretical on new builds.However, a knowledge of the electrical equipment on the platform 10 willenable a valid spectral model to be generated.

This analysis is used to identify the VLF or ELF frequency band in whichthe system will operate, and to enable frequency excision requirementsto be defined. These are largely software modifications and could beinstalled either during manufacture or by intervention with the systemonce installed.

It is anticipated that the noise environment will change over thelifetime of the pipeline. This should not cause a problem if the systemhas an adequate link budget margin built in.

The pipeline configuration analysis will establish the signal powerinput required at the platform by considering loss mechanisms and noisesources. The maximum transfer range of the system is largely governed bythe quality and integrity of the cathodic protection measures embodiedin the pipeline construction and, in essence, the better the insulationof the pipeline 12,14, the further the system can transfer data.

A data transfer installation may consist of a single master stationcontrolling up to 32 sub-sea data transfer units 36.

The architecture proposed for the system platform 10 and sub-seaprocessors 55 is structured to enable access to an array of operatingdata parameters which are unique to the oilfield installation. Theseinclude:

Frequency of known interference sources (Hz);

Identification address of the DTU (4 digit);

Identification number for each transducer being monitored (2 digit);

Changes from the default sampling rate;

Changes from the nominal data transfer interval; and

Changes to the default alarm ranges for each transducer.

The memory location is accessible to the platform operator, andindividual data items stored may be updated at any time over the unit'soperating life.

The transducer suite interface will accommodate up to:

16 bit analogue devices; and

16 single bit digital devices.

An interface unit will digitise the input range of each analogue devicefor temporary storage in the system memory. The memory will accommodate400 data samples of each transducer in a circular buffer. All datapoints include the transducer identity and time of sampling. The DTUmemory map is presented in FIG. 16.

The sub-sea elements of the Data Transfer System can operate in eitherself powered or externally powered configuration.

External power is considered to be a possibility when the system isinstalled in a back-up role, perhaps providing a safety facility toenable continued telemetry from sub-sea facilities in the event ofumbilical degradation. With external power available, there are noconstraints on the duty cycle of the system.

The system will operate in a self-powered configuration when located ona christmas tree which has no electrical supply. Power is produced fromthe thermo-electric generator 39 located on the oil flowline 14 as hasbeen described above with reference to FIGS. 11, 14 and 15.

The DTU battery pack 61 is sized to provide 100 hours of operation priorto any energy top-up from the PGU 39. This period is intended to providetelemetry from the tree 15 during the start-up procedures.

Clearly, the PGU 39 reduces the internal energy of the oil by a smallamount in this process, although the temperature reduction is a functionof the flow rate and temperature gradient at that location. If thistemperature reduction is seen as significant, an additional insulationcoating could be applied to the local pipeline 14 so that the net heatloss at the tree site is unchanged.

The system is provided with a real time clock which is the only elementof the system in continuous operation between the date of installationand the scheduled well start-up. This minimises the power demand of thesystem prior to the availability of the power generation unit 39, orexternal power supply.

The clock is part of a system initialisation circuit. The clock will beset during the system software configuration for the particularinstallation.

It is reasonable to predict the elapsed time between installation on thesea bed and the well start-up, and the clock will count down a dormancyperiod of up to 9000 hours using a Real Time Clock (RTC). At the zerohour, the system will be switched on. The clock will proceed to countfor a period of up to 2 hours during which the system is listening for atransmission bearing its address code.

If no signal is received at the end of this period, the system will shutdown and the clock will count up to a period of 168 hours (7 days userconfigurable). At this time the system will again turn on and listen fora signal. This "H" hours on, "D" days off cycle will be repeated untilcommunication is established, or power is available from the powergeneration system.

If, at any time, the initialisation circuit detects power is available,the system will enter its normal data sampling mode at the defaultsampling rate. Once communication is established, the system is undercontrol from the platform 10.

The first instruction sent from the platform 10 to a DTU 36 has threefunctions:

enable its transducer monitoring system and begin to assemble the datastream for transfer to the platform 10, or

re-set the dormancy/wake-up cycle parameters, and

synchronise the DTUs 36 with the master station 11.

Following a request for data, the Master Station waits for a replymessage.

The first instruction from the platform 10 advises the signal processor55 of any variations to the default sampling rate embodied in itssoftware. The default sampling rate is once/60 minutes. The operator canmodify the sampling rate as required at any time during the life of theDTU.

Normal data transfer is an automatic procedure. At regular intervals,the master station 11 launches the carrier on the pipeline 12,14 andimposes the digital address code and a data send command for one DTU 36.

After a defined period, the master station 11 curtails data transmissionand listens for an upcoming signal. The sub-sea DTU 36 then follows anidentical procedure and transfers the requested data.

On receipt, the master station 11 checks that the message is completeand valid. If it is not, then a repeat request is made otherwise theMaster Station 11 moves on to request data from the next DTU 36.

All upcoming data streams are stored on floppy disc and may be assessedby graphical print-out, display or other analysis mediums which arecommercially available. The control unit 24 will alert the operator whendisc capacity exceeds 70%.

Normal sequential data transfer procedures may be interrupted by theoperator at any time to allow special data transfer.

Special data transfers are initiated by the operator. The master station11 is instructed to launch the carrier and impose on it:

The address of the DTU 36 of interest;

The address of the transducer(s) within that DTU 36;

Any step changes to the default sampling rate;

Any changes to the alarm range settings;

Data transfer command for defined data stored by circular buffer of theDTU 36.

Once sent, the master station waits for reply from the addressed DTU 36.This operation is terminated by master station operator command.

The master station 11 then returns to normal data transfer modeautomatically.

Each sub-sea DTU 36 incorporates an alarm detection routine whichmonitors:

current transducer value against user defined (or default) alarm range;and

recent transducer value trends for convergence with a user defined alarmrange.

If either condition is detected, an alarm interrupt code is transmittedduring the interrupt time domain of any current signal on the network orat any time if no transmissions are currently taking place.

This interruption causes the master station 11 to interrogate thealarmed DTU 36 and display an alarm warning on the platform 10; if morethan one DTU 36 enters alarm condition then a schedule of interrogationsis defined by the master station 11. In general, a reduced set oftransducer samples will be transferred during alarm conditions toexpedite the data availability at the platform 10.

The sub-sea elements of the data transfer system are configured for bothnew build pipelines or retrofit to an existing pipeline.

The DTU 36 should be sited to ensure that insulation of the pipeline12,14 from the earth path return (i.e. the well casing) is notcompromised. Cable runs will link each unit 36 to:

the transducer arrays;

either the external power supply or the power generation unit 39; and

the upstream and downstream connections of the pipeline insulation Joint21.

For retro-fit on an existing tree 15, the sub-sea components of thesystem may be configured as a pipeline spool-piece. This assembly wouldincorporate the PGU 39, DTU 36, wiring harness and insulating joint 21within a structural framework to facilitate handling and installation. Apossible configuration is shown in FIG. 17.

During installation at the tree site, the transducer wiring harnesswould be stowed in a vented containment.

It is envisaged that the spool-piece configuration would be specific tothe installation site.

For new completions, the sub-sea system components may be mounted on thestructure of the tree 15, enabling integrated testing of the tree 15 andthe system on land.

During data transfer the earth path return of the well 16, and sea pathreturn of the platform 10, are energised and an electro-magnetic fieldis created. Because of the large surface areas at connections 19 and 23between well 16 and earth and platform to sea, the field strength at anylocation is small. Accordingly, the hazard to divers is considered to beno worse than for current sub-sea installations. Field strengths fromthe system will be comparable with those produced during sub-sea weldingor at a sacrificial anode.

The crude product will be exposed to a low field strength alternatingpotential during transmissions. The effect, thus, will vary betweencrude products depending on their electrical resistance.

Fields around the sea path connection at the platform 10 areintermittent and of lower intensity than those of current sacrificialanodes. It is reasonable to infer that the system will have no moreeffect on marine life than current cathodic protection systems.

The PGU 39 will reduce the temperature of the crude product as it passesthrough it. If considered significant, this effect may be countered byprovision of an insulation jacket to clad an equal surface area of thelocal flowline 14, such that the net heat loss is unchanged from currentinstallations.

Structural steel guards are provided around all units of the system toensure that transport and installation loads are accommodated withoutrisk to the equipment.

Component redundancy is provided for the PGU 39 (4 banks of 60 devices)and the battery pack 61 (4 individual units 58). These will be installedto ensure graceful degradation in the event of component failures.

The invention may be operated to provide a condition monitor for theinsulation of the pipeline. This may be done by determining the timehistory of errors in the carrier signal or basic link by comparing thebasic link data with the processed data. Comparison of the basic linkdata with the processed data will produce an error count which dependson noise environment and signal attenuation due to leak paths.Alternatively it may be possible to detect corrosion effects by longterm monitoring of input characteristic impedance of the pipe. Theanalysis can be extended to generate a Link Monitor Index. A healthmonitor module within the software of the system may be arranged toperform this task for all data transfers. Given that the noise leveleffects tend to average out over time, any trends (or step changes) inthe value of the Link Monitor Index will be a reflection of the changesin the index and changes in the slope of the index time history. Eithercondition would trigger an alarm routine from the master station 11.

FIGS. 18 and 19 illustrate a system for transferring pressure andtemperature data from a down hole location of a sub-sea well, in whichthe invention is embodied. FIG. 19 shows that insulation cored centringrings 76 are fitted to the production string 77 for providing insulationbetween the string 77 and the well casing 78.

In carrying out the invention a VLF or ELF EM signal is launched ontothe production string 77 at the wellhead 16. The string 77 would need tohave reasonable continuity, as well as be insulated as described, tominimise the power required at the wellhead 16 and to minimise currentleak paths to the surrounding water, or kill fluids in the well. Afurther reason for providing insulation for the string 77 is the needfor establishment of a reliable current return path.

The basal impedance of the string 77 and of its noise environment wouldneed to be assessed before the instrumentation module, which may beelectrically passive, is insulated in order to enable the signalstrength and optimum frequency to be identified. The instrumentationmodule is packed in an annular structure and provides the followingsystems:

A down hole production string interface;

Pressure and temperature instrumentation including a Bourdon tube forpressure measurements and a bi-metallic strip for temperaturemeasurements;

Tuned circuit installations; and

A casing interface.

The down hole production string interface consists of a tuned circuitwhich is energised by the carrier modulation induced on the productionstring 77. Movements of the Bourdon tube and the bi-metallic strip arearranged to vary two separate circuits to provide resonant frequencieswhich relate to temperature and pressure. A frequency sweep at thewellhead will establish the resonant spikes.

The data available at the wellhead 16 could be transferred to theplatform 10 either by an umbilical cable or by a sub-sea data transfersystem as has been described above with reference to FIGS. 1 to 17.Either form of Power Generation Unit 39 described above would be capableof powering the production string interface coils to provide down holesampling.

We claim:
 1. A pipeline system comprising a metal pipeline having a coating of insulating material which, in conjunction with sacrificial anodes electrically connected to the pipe provides cathodic protection, there being signal generating means adapted to be coupled to the pipeline and operable when so coupled to apply data to be transmitted in signal form to the pipeline for conveying along an electrically conducting channel formed by the metal pipeline, wherein the improvement comprises the signal generating means being operable to generate as the signal a pulsed electro-magnetic signal which comprises changes of voltage level in a very low frequency range (VLF and below), and further including means operable to use data transmitted via the channel provided by the pipeline for monitoring the condition of the insulation of the pipeline.
 2. A pipeline system according to claim 1, adapted for the transfer of data between sub-sea facilities and a production platform of a sub-sea oil pipeline installation, the data to be transferred comprising instrumentation status information from any point on the pipeline installation, and the pipeline which forms the electrically conducting channel comprises a pipe for carrying the fuel/water between a well-head and the production platform, wherein a master station is provided for managing signalling in the system and for location on the platform, the master station being adapted so that the signal processing management takes place in one of three modes, viz. normal data transfer, special data transfer and alarm interrupt; means being provided whereby normal and special data transfers are initiated by the master station and alarm interrupts are initiated by any sub-sea unit which detects an alarm condition.
 3. A pipeline system according to claim 2 wherein a thermo-electric generator is provided for sub-sea locations which do not have mains power available.
 4. A pipeline system according to claim 3 wherein the thermo-electric generator has no moving parts and employs solid state arrays of thermo-electric devices which are sealed.
 5. A pipeline system according to claim 3 wherein the power generation unit is in an in-line configuration.
 6. A pipeline system according to claim 3 wherein the power generation unit is clamped around the pipeline.
 7. A pipeline system according to claim 2 wherein all signal sequences incorporate an interrupt time domain to facilitate near real-time transfers of the alarm information.
 8. A pipeline system according to claim 7 wherein a sub-sea data transfer unit which detects an alarm or a history of errors in the signal indicative of a future alarm condition in its transducer array, is adapted to transmit an alarm identity during the interrupt time domain of any ongoing transmission.
 9. A pipeline system according to claim 2 wherein a flow turbine generator is provided for a sub-sea injection well which has no mains power available.
 10. A pipeline system according to claim 2 wherein the data concerns conditions at a down hole location of a sub-sea well and in operation of the system is transferred from that location along the structure of the production string of that well.
 11. A pipeline system according to claim 1, wherein the signal generating means are operable to generate the electromagnetic signal in the very low frequency range (VLF and below) so that it comprises changes of voltage level oscillating about the DC voltage level of the pipeline so that the mean level of the signal is substantially the DC voltage level of the pipeline.
 12. A pipeline system comprising a metal pipeline having a coating of insulating material which, in conjunction with sacrificial anodes electrically connected to the pipe provides cathodic protection, there being signal generating means adapted to be coupled to the pipeline and operable when so coupled to apply data to be transmitted in signal form to the pipeline for conveying along an electrically conducting channel formed by the metal pipeline wherein the improvement comprises the signal generating means being operable to generate as the signal a pulsed electro-magnetic signal which comprises changes of voltage level in the very low frequency range, the signal generating means being operable to generate the electro-magnetic signal in the very low frequency range so that it comprises changes of voltage level oscillating about the DC voltage level of the pipeline so that the mean level of the signal is substantially the DC voltage level of the pipeline.
 13. A pipeline system according to claim 12, adapted for the transfer of data between sub-sea facilities and a production platform of sub-sea oil pipeline installation, the data transferred comprising instrumentation status information from any point on the pipeline installation, and the pipeline which forms the electrically conducting channel comprises a pipe for carrying the fuel/water between a well-head and the production platform.
 14. A pipeline according to claim 13 wherein the generated signal is in the extremely low frequency range below the audio range.
 15. A pipeline system comprising a metal pipeline having a coating of insulating material which, in conjunction with sacrificial anodes electrically connected to the pipeline provides cathodic protection, and signal generating means adapted to be coupled to the pipeline and operable when so coupled to apply data to be transmitted in signal form to the pipeline for conveying along an electrically conducting channel formed by the metal pipeline, wherein the improvement comprises the signal generating means being operable to generate as the signal a pulsed electro-magnetic signal which comprises changes of voltage level in a very low frequency range (VLF and below), and wherein means are provided which are operable to use data transmitted via the channel provided by the pipeline to monitor a characteristic impedance of the pipeline and thereby monitor the condition of the insulation of the pipeline.
 16. A pipeline system comprising a metal pipeline having a coating of insulating material which, in conjunction with sacrificial anodes electrically connected to the pipeline provides cathodic protection, and signal generating means adapted to be coupled to the pipeline and operable when so coupled to apply data to be transmitted in signal form to the pipeline for conveying along an electrically conducting channel formed by the metal pipeline, wherein the improvement comprises the signal generating means being operable to generate as the signal a pulsed electro-magnetic signal which comprises changes of voltage level in a very low frequency range (VLF and below), and wherein means are provided which are operable to use data transmitted via the channel provided by the pipeline to determine a time history of errors in a carrier signal and thereby monitor the condition of the insulation of the pipeline. 